The present invention relates to a drilling stabilizer for guiding and stabilizing a drill bit and a drill string for boring an oil or natural gas well.
An oil or natural gas well is drilled by rotating a drill bit connected to the surface through a drill string. A drilling stabilizer is used to guide the drill bit at the tip and the drill string and stabilize their movement. High strength and high wear resistance are required for such a stabilizer because it collides hard against rocky wall of the drilled hole.
FIG. 6 shows a conventional blade-welded type stabilizer having its steel blades 2a welded to the outer periphery of its body 1a. To improve wear resistance of the blades 2a, a plurality of buttons 3 made of cemented carbide are pressed into the surface of each steel blade 2a, and the blades 2a are welded to the outer periphery of the stabilizer body 1a.
As shown in FIG. 7, for a stabilizer having blades 2b integral with the stabilizer body 1b, cemented carbide inserts 4 are arranged at a high density on each blade 2b, and the blades 2b are brazed to the surface of a steel blade body 1b by use of a hardened brazing filler.
The present inventors disclosed composite materials comprising a steel substrate and cemented carbides bonded by current pressure sintering with gradient composition in unexamined Japanese patent publication 9-194909, and in a Japanese technical magazine "Powder and Powder Metallurgy" vol. 43 (1996), p472 and vol. 44 (1997), p269. These are formed by laminating cemented carbide layers having different compositions. The cemented carbide layer adjacent the steel substrate has a high cobalt content while the cemented carbide layer on the surface side has a low cobalt content to suppress thermal stresses during sintering resulting from a difference in thermal expansion coefficient between the cemented carbide and steel, and to provide a high-hardness outer cemented carbide layer and a high-toughness inner cemented carbide layer. Thus, the inner layer and the surface layer perform separate functions, thereby providing an unprecedented cemented carbide structure that is high in both wear resistance and resistance to chipping.
It is also disclosed that by adjusting the laminate structure, a suitable compressive residual stress is introduced into a surface portion, so that it is possible to improve the resistance to chipping of the cemented carbide portion.
The present inventors also disclosed in Collection of Articles published by Petroleum Technology Society, 1996, P103 that high performance can be expected by using the cemented carbide/steel material of a gradient composition in the blade-welded type stabilizer shown in FIG. 8 and by bonding the above-mentioned laminate 5 over the entire top surface of each blade 2c provided on the stabilizer body 1c.
In the conventional arrangement in which cemented carbide buttons are buried in a steel blade, since steel and cemented carbide widely differ in wear resistance, the steel portion, which is inferior in wear resistance, tends to wear more quickly in use than the cemented carbide portions, so that the cemented carbide buttons tend to protrude from the steel. This increases the possibility of chipping of the cemented carbide buttons, resulting in the shortening of life. Further, in the conventional type, the corner of the blade has to be necessarily formed from steel, so that the blade ridges tend to be damaged which collide against the wall of the drilled hole.
Also, in the conventional button-buried type, in order to improve wear resistance, it is necessary to increase the number of cemented carbide buttons pressed into steel. But since the buttons have to be supported by the steel substrate, the number of buttons that can be buried in the substrate is limited. About 30% is the upper limit in the ratio of area of buttons to the entire surface area of the steel substrate. Thus, to improve wear resistance, that of the cemented carbide buttons is important. For this purpose, cemented carbide containing 7-9% Co is used. But cemented carbide having this composition tends to chip during use. That is, such a cemented carbide is low in resistance to chipping.
Further, the blades 1a-1c of the conventional stabilizers have a rectangular shape with one side longer than the other side. In order to form blades having such a rectangular shape in a current pressure sintering apparatus, a large facility is needed, so that the manufacturing cost tends to be high.
Further, if a cemented carbide member is bonded to the entire surface of the blades of the stabilizer except its sides, i.e. the entire top surface, the outer ridges of the blades will be of cemented carbide instead of conventional steel, so that the ridges, which collide first against the wall of the drilled hole, tend to chip.
Also, different loads act on different portions of the blades of the stabilizer, and during use of the stabilizer, lower portions tend to be damaged more severely than upper portions, so that the lower slope tends to be damaged most severely while the upper slope tends to be the least damaged. Since different portions are damaged to different degrees, it was desired to improve durability of blades by remedying such uneven damage. In the cemented carbide button-buried arrangement, according to the degree of damage, the number of cemented carbide buttons is changed to increase the surface area ratio of cemented carbide at portions that tend to be damaged. But the adjustable range is limited.
The blades had a top surface comprising a central curved surface and slopes at both ends. Since the slopes are flat and the ridges forming the boundaries between the central curved surface and the flat slopes at both ends are angular, stresses tend to concentrate on the ridges. In the case of steel, this is no problem. But if a cemented carbide member is bonded to the steel substrate over its entire surface, it tends to be damaged due to stress concentration on the ridges.
Further, in future, stabilizers will have to be used in increasingly hostile environments because they are expected to be used not only to dig deeper holes but many oblique and even horizontal holes from a single rig besides vertical holes to dig oil and natural gas. Thus, it is desired to further improve wear resistance and strength of the blades of stabilizers because these factors greatly influence the life of the stabilizers.
An object of this invention is to provide a high-performance drilling stabilizer that is high in both wear resistance and strength.